Process for integrated evaporation and gasification of spent liquor from cellulose cooking

ABSTRACT

The invention relates to a process for integrated evaporation and gasification of spent liquor from the production of cellulose, the gasification taking place in a reactor (1) at a pressure of from atmospheric pressure up to about 150 bar and at a temperature of about 500°-1600° C., by partial combustion of the spent liquor, a gas phase and a phase of solid and/or smelted material being formed in the reactor and being removed therefrom. The invention is characterized in that the combustion gas obtained from the reactor (1) by partial combustion of the spent liquor is brought together in direct contact with spent liquor, which may have been pre-evaporated, in a first stage--for example a venturi scrubber--, with alkali metal compounds, etc., which have been gasified in the combustion gas, being returned to the reactor together with the concentrated spent liquor, after which the combustion gas, preferably in at least one additional stage, is permitted, directly or indirectly, to give off additional evaporation heat to the spent liquor.

This application claims benefit of international applicationPCT/SE94/01,046, filed Nov.8. 1994

This application claims benefit of international applicationPCT/SE94/01,046, filed Nov. 8, 1994.

TECHNICAL FIELD

The present invention relates to a process for integrated evaporationand gasification of spent liquor from the production of cellulose., theevaporation and gasification being integrated in the same system whileutilizing the thermal energy in the combustion gases after thegasification.

STATE OF THE ART

A number of systems are already known for evaporating and gasifyingspent liquors from cellulose cooking. The spent liquor or the blackliquor, as it is usually termed, contains both organic substances andinorganic chemicals, principally alkali, in aqueous solution. Thechemical energy which is present in the black liquor is utilized by theorganic material being burnt at the same time as the inorganic chemicalsare extracted so that they can be re-used in the process. For it to bepossible to burn the organic material partially and to recover thechemicals, the major part of the water in the black liquor must beevaporated in advance so that a concentrated spent liquor can besupplied either to a recovery boiler or a gasification reactor.

Evaporation of black liquor is normally effected in several stages, forexample in five evaporators coupled in series. For example, evaporationis carried out using three evaporation apparatus coupled in series andthe intermediate liquor produced in these apparatus is then supplied toa pair of evaporation apparatus coupled in series for final evaporation,with thick liquor being obtained from these latter apparatus. Heat forthe evaporation is supplied by means of admission steam, whichexpediently has a temperature of 140° C. and which leaves the system ata temperature of 60° C., for example. To make this possible, use is madeof vacuum pumps.

Other systems for concentrating black liquor involve pressurizing theliquor after preliminary evaporation, with the liquor then being allowedto expand in an expansion vessel, whereupon steam vaporizes. Such asystem is described, for example, in the U.S. Pat. No. 4,909,899.

A further system, which utilizes venturi scrubbers and separators, isdescribed in the Swedish patent 318 469. In this system, use is made ofvacuum fans, and exhaust gases are employed at a low temperature whichis initially approximately 280° C. and which falls to 74° C. at theoutlet for the exhaust gases.

TECHNICAL PROBLEM

The problem with the known solutions is that use must either be made ofa supply of admission steam or else the process must be carried outusing large installations at relatively low temperature.

There is often a shortage of capacity, both for the spent-liquorevaporation and the recovery stage, the latter being either in the formof a recovery boiler or a gasifier. If a gasifier is used, there is thususually the need for an equivalent capacity-increasing effect, asregards spent-liquor evaporation, to be achieved in a manner which iscost-effective, energy-saving and environmentally acceptable. There isalso the need for the spent liquor to be subjected to an additionalconcentration-increasing stage prior to the gasifier, at the same timeas gasified alkali metal compounds are separated out of the combustiongases as rapidly as possible and returned to the reactor.

SOLUTION

In accordance with the present invention, a process has therefore beendeveloped for evaporating and gasifying spent liquor from the productionof cellulose, which process integrates these two process steps, thegasification taking place in a reactor at a pressure of from atmosphericpressure up to about 150 bar and at a temperature of about 500°-1600°C., by means of partial combustion of the spent liquor, a gas phase anda phase of solid and/or smelted material being formed in the reactor andbeing removed from this reactor, which process is characterized in thatthe combustion gases from the reactor are brought together, in directcontact, with the spent liquor in a first stage in a venturi scrubber,static mixer or the like, after which the combustion gases arepermitted, preferably in at least one additional stage, directly orindirectly to give off additional evaporation heat to the spent liquor.

In accordance with the invention, it is expedient for the additionalstage to comprise a film evaporator having indirect heat transfer.

In accordance with the invention, the temperature of the combustiongases is permitted to fall from, preferably, reactor temperature, forexample 950° C., down to less than 300° C., preferably less than 250° C.and more preferably to approximately 200° C.

In accordance with the invention, it is expedient for, for example,pre-evaporated spent liquor to be supplied to the second stage in thefilm evaporator and for a part of the evaporated liquor in the filmevaporator to be recirculated and a part to be supplied to the firststage, from which concentrated spent liquor is supplied to the reactor.

In accordance with the invention, the additional stage can, instead,comprise a cyclone evaporator having direct heat transfer between thecombustion gases and the liquor.

In accordance with the invention, incoming liquor is supplied partly tothe first stage and partly to the second stage in one or more banks ofnozzles, and concentrated liquor is both recirculated and in partsupplied to the reactor.

In accordance with the invention, at least a part of the H₂ S content ofthe combustion gases can be recirculated to the reactor (the wholequantity or part of the quantity).

In accordance with the invention, the pressure in the reactor shouldexceed 3 bar, preferably 8 bar and more preferably 20 bar.

DESCRIPTION OF THE FIGURES

The invention will be described in more detail below while referring tothe attached drawings, in which

FIG. 1 diagrammatically shows an embodiment of the invention, and inwhich

FIG. 2 diagrammatically shows a preferred embodiment of the invention.

PREFERRED EMBODIMENTS

FIG. 1 shows a reactor 1 for gasifying thick liquor by means of thepartial combustion of the organic substances. The design of the reactorindicates a quenching device, with the hot gas and smelt phases whichare leaving the reactor being cooled in a connecting liquid bath whichis arranged beneath the reactor space itself. The pressure in thereactor is, for example, 25 bar and the temperature of the gases 4leaving the quench is then of the order of size of 200° C. Gasificationreactors of this type are well known and will not therefore be describedin more detail here. At its upper part, the reactor is fed with thickliquor, which is supplied via a pipe 2. Recovered chemicals are removedfrom the bottom part via a pipe 3. The reactor can be of a differenttype and comprise so-called hot-gas separation, signifying that thecombustion gases which are fed into the subsequent stage 5 have atemperature of, for example, 950° C. In order to accomplish hot-gasseparation, use is preferably made of a liquid film cyclone (see WO93/01890 or, for example, the method described in WO 93/02249).

In accordance with the invention, the hot, pressurized combustion gasesflow, via the pipe 4, into a venturi scrubber or a static mixer, or acombination of these elements. In this latter unit 5, the combustiongases meet black liquor from the pipe 6, which black liquor has alreadypassed through one or more evaporation stages. In this unit, thecombustion gas is permitted to have direct contact with the black liquorand evaporates the latter to a desired level which makes it suitable forbeing introduced into the reactor. The evaporated liquor is removedthrough the bottom part and is permitted to flow into the pipe 2 via abuffer 7.

The combustion gas flowing from the unit 5 through the pipe 8 is thenpermitted to flow into a further evaporator 9, which, in the presentcase, is a falling-film evaporator. In this falling-film evaporator, thecombustion gases, which are pressurized, do not come into direct contactwith the liquor. The liquor, for example in the form of an intermediateliquor having a dry matter content of 40% and a temperature ofapproximately 70° C., is supplied to the falling-film evaporator fromthe top through the pipe 10. This liquor is combined with a moreconcentrated liquor, from the pipe 11, which is recirculating in thefalling-film evaporator. As can be seen in the figure, the pipe 11branches into a branch 6 which has already been mentioned.

The combustion gas which leaves this second stage in the evaporationprocess, and which is still pressurized, is conducted out, at a lowertemperature, through the pipe 12 for further use. Liquor vapours whichhave condensed out from the combustion gas are conducted out through thepipe 13.

In the upper part of the falling-film evaporator, pressurized liquorvapour is conducted out through the pipe 14 at a temperature whichcorresponds to the pressurization, for example 3 bar and above. Inaccordance with the invention, the liquor vapour can be used, forexample, for pre-evaporating incoming black liquor. This liquor vapourthus replaces admission steam, which is normally added at the finalevaporation. The pre-evaporation refers to evaporation of mixed liquorhaving a dry matter content of the order of size of 20%, and brings thisliquor up to a so-called intermediate liquor having a dry matter contentin the range of 40%, which liquor is fed in through the pipe 10.

The thick liquor which is supplied to the reactor through the pipe 2,and whose evaporation has been finished, has a dry matter content of,for example, 80%.

As has been mentioned above, the combustion gases come into directcontact, in the venturi scrubber or the static mixer, with the blackliquor which has been concentrated in previous stages. This avoids theuse of heat-transfer surfaces which are difficult to handle and whichcan easily become encrusted. Alkali and alkali metal compounds which arepresent in gasified form or in solid form as "submicrons" are alsoabsorbed from the combustion gases in this stage and returned to thegasification reactor by means of the thick liquor.

FIG. 2 shows a preferred embodiment of the present invention. In thisembodiment, concentrated black liquor is not removed from the venturiscrubber 5 but is transferred, together with the combustion gases, intoa cyclone evaporator 15. In addition to combustion gases, the venturiscrubber 5 is also supplied with intermediate liquor, having a drymatter content of approximately 40% and a temperature of approximately70° C., through the pipe 10, which pipe 10 also has a bifurcation 16which opens out in nozzles for spraying intermediate liquor into theupper part of the cyclone evaporator 15. Black liquor, which isrecirculating through the conduit 17, is also sprayed into the upperpart of the cyclone evaporator 15. In accordance with the invention,this conduit 17 also has a bifurcation 18 which sprays liquor into thehot combustion gases from the venturi scrubber 5. A film of black liquorruns downwards continuously along the walls of the cyclone evaporatorand collects at the bottom of the container. After that, theconcentrated liquor is removed through the pipe 2 and supplied to thereactor 1.

The combustion gases departing from the top of the cyclone evaporatorexpediently maintain a temperature of 200° C. and are conducted out viathe pipe 19. This pipe 19 has to pass through a heat exchanger 20, whichcondenses out the moisture content of the combustion gases. Thedeparting, pressurized combustion gas is now treated, inter alia, withregard to the presence of alkali metal compounds.

The pressurized combustion gas can also be treated in a so-called `acidgas removal` system (AGR) 21. In this system 21, H₂ S is extracted andthen concentrated and recirculated to the gasification reactor. Thereaction equilibrium in the reactor is thereby displaced so that theformation of carbonate compounds in the inorganic smelt phase issuppressed to the greatest possible extent (Pat. No. WO 93/12288).

By means of the present invention, exemplified for black-liquorgasification, treatment of incoming spent liquor is integrated withgasification by means of utilizing the thermal energy of the combustiongas while simultaneously returning alkali metal compounds, etc., whichare gasified in the combustion gas, to the gasifier together withconcentrated black liquor.

Only two embodiments are shown, but further embodiments are conceivableand the invention is not therefore limited to these two embodiments butcan be varied in different ways within the scope of the patent claims.For example, use can be made of other spent liquors, such as spentbleaching liquors, biosludge, etc., within differing concentrationranges. In addition, it will be evident to the person skilled in the artthat the second evaporation stage, in accordance with the examplesshown, constitutes only one preferred embodiment and that the heatenergy remaining in the hot gases after the first stage can be utilizedin another manner, for example for producing admission steam. A lowerpressure is preferably used in the first stage in conjunction withquench than in conjunction with hot-gas separation, since the availabletemperature drop is lower for the quench alternative.

Naturally, additional evaporation stages can be coupled together.

I claim:
 1. A process for the integrated evaporation and gasification ofspent liquor from the production of cellulose using a reactor in whichthe pressure is maintained in a range of above 3 bar to about 150 barand a temperature of up to 1600° C. by partial combustion of the spentliquor which forms a combustion gas phase and a phase of solid orsmelted material, removing the material from the reactor and separatingthe gas phase from any remaining phase, then, in a first stage, directlycontacting the combustion gas phase with the spent liquor in a mixingmeans and then using the heat content of the combustion gas phase in atleast one additional phase, said additional phase constituting anadditional evaporation phase and comprising a film evaporator havingindirect heat transfer.
 2. The process as claimed in claim 1 wherein thespent liquor is pre-evaporated and is supplied to the additional stagewhich includes a film evaporator, a part of the evaporated liquor in thefilm evaporator is recirculated and a part is supplied to the firststage and including the step of supplying concentrated spent liquor fromthe first stage to the reactor.
 3. The process as claimed in claim 1wherein the combustion gas contains H₂ S and including the step ofreturning the H₂ S to the reactor.
 4. The process as claimed in claim 1wherein the separation of the gas phase is a hot-gas separation wherethe temperature in the combustion gas is permitted to fall to less than300° C. in the additional stage.
 5. The process as claimed in claim 4wherein the temperature in the combustion gas is permitted to fall toless than 250° C.
 6. The process as claimed in claim 4 wherein thetemperature in the combustion gas is permitted to fall to less thanapproximately 200° C.
 7. The process as claimed in claim 1 including thestep of using a venturi scrubber as the mixing means.
 8. The process asclaimed in claim 1 including the step of using a static mixer as themixing means.
 9. A process for the integrated evaporation andgasification of spent liquor from the production of cellulose using areactor in which the pressure is maintained in a range of above 3 bar toabout 150 bar and a temperature of up to 1600° C. by partial combustionof the spent liquor which forms a combustion gas phase and a phase ofsolid or smelted material, removing the material from the reactor andseparating the gas phase from any remaining phase, then, in a firststage, directly contacting the combustion gas phase with the spentliquor in a mixing means and then using the heat content of thecombustion gas phase in at least one additional phase, said additionalphase comprising a cyclone separator having direct heat transfer betweenthe combustion gas and the liquor.
 10. Process according to claim 9,characterized in that spent liquor is supplied partly to the first stageand partly to the additional stage in one or more banks of nozzles, andin that concentrated liquor is partly recirculated and partly suppliedto the reactor.